The invention is related to a flat-sheet die for the extrusion of flat sheets, having an upper and a lower die lip, which define the outlet gap above and below.
Flat-sheet dies of this kind are employed in plastic sheet production. At least one of the two die lips is embodied as flexible in order to be able to subject the outlet gap to fine adjustment and regulation in accordance with the measurement data obtained. The adjustable lip as a rule is formed by tapering the material, by way of which, via adjustable-length actuators, a corresponding bending load is exerted upon the at least one adjustable sealing lip in order to thereby bring about the adjustment.
A multitude of publications related to flat-sheet dies have already become known, in which the outlet gap can be enlarged or made smaller for example by means of actuators, which can be heated and thereby changed in length. The tool lip which regulates the gap can be provided in other embodiments as well, for instance by means of a field translator, which is based upon the piezoresistive effect.
For example, EP 0 367 022 A2 has made known an extrusion slot die, which includes a multitude of passive actuators, which influence a flexible band; furthermore a number of laminas, which are movably disposed between the band and the flexible lip, are provided for the introduction of the actuating force from the flexible band onto the lip. As a result, the narrowest-band regulation of the outlet gap possible should be enabled, without increasing the number or the spacing of the active actuators, which could not be done anyway for lack of sufficient installation space.
DE 37 31 961 A1 has made known a device for adjusting the gap of a die by means of a thermobolt, which device furthermore includes an air cooling for the more rapid cooling of the thermobolt and hence for the contraction of the actuator.
To that end, axial cooling conduits are provided in the thermobolt, by way of which, via a connectable compressed air hose, a corresponding quantity of cooling air can be adjusted and hence the cooling capacity can be varied. The cooling air is generally compressed air. Here as well the thermobolt itself is embodied once again as a standard compression-tension element.
Additional actuators for the outlet gap, having cooling devices are also known from, among others, EP 0 383 418,
DE 36 28 974 C1, EP 0 257 233 B1, EP 0 256 490 B1, and EP 0 229 680 B1. These publications likewise include cooling conduits, through which as a role air can flow through in a controlled manner as a coolant. In EP 0 229 680 B1, it is also described that a fluid, e.g. water, can circulate as the cooling medium.
The flat-sheet dies according to the prior art, though, have disadvantages, which in the production of plastic sheets are relevant to the winding process.
Thus, a general problem for systems of this kind lies in the fact that at the winding station, as the winding thickness of the plastic sheet increases, thick or thin places result from existing thickness variations of the plastic sheet and do not allow rotally constant winding without any thickening or protruberance over the entire width of the plastic sheet.
Later generations of flat-sheet dies have indeed already made possible certain improvements with regard to the desired sheet quality (tighter thickness tolerances). Nevertheless, the results produced before today are no longer satisfactory.
The main problem lies in the range of influence of an actuator, which is too broad for an optimal sheet quality; this is due on the one hand to the wide spacing of the active actuators of over 25 mm, for geometrical reasons of the apparatus, and on the other hand to a massive embodiment of the flexible lip and of the bending line resulting from it. Particularly in sheets (such as BOPP), i.e., Biaxially Orientated Polypropylene which are stretched widthwise, the range of influence with reference to the final film lies in an actuator spacing of from 28 mm to over 230 mm; by means of the bending line of the flexible lip, at least two adjacent zones on each side are influenced. Consequently, small thickness tolerance variations with a width of 100 mm, for example, which can accumulate upon winding until they form so-called "piston tings", can no longer be sufficiently corrected.
A further problem up until now lay in the fact that the adjusting speed of the actuators is too slow, with the result that significant sheet lengths are extruded and wound until a corresponding readjustment can be carded out only after there is an ascertained or measured deviation of the sheet thickness from a desired measure.
Finally in flat-sheet dies according to the prior art, it has turned out that the adjusting unit should be as insensitive as possible to the environmental influences and should have as great a reliability as possible.